It may be useful, and it is certainly interesting, to be able to tell the time at night without the use of watch or clock, and it can be done if you know a little about the stars. The simplest way is to imagine the night sky to be a rather peculiar kind of clock-face. (Click on diagram.)
On the circle of sky overhead imagine the hours to be written round the edge, as they are in the diagram; it is a 24-hours clock, with 12 midnight at the North point, 12 noon due South, 6 A.M. to the West, and 6 P.M. to the East. All the A.M. hours are on the West or left side, all the P.M. hours on the East or right.
Then imagine that it has one large hand moving round the face, but moving backwards, or counter-clockwise, or from 12 midnight towards 6 A.M. Now you need to remember just one Zero, or starting-point, and from that you can always calculate the time. Your Zero is the date March 21st, because at midnight on that date the hand of your clock will always point to midnight on the imaginary star-clock; i.e., due North.
But where is this hand? Well, at Zero three particular stars are always in a straight line pointing North, and they are all easy to find. One is the Pole Star, which, as you know, is the tail star of the Little Bear. The second is the star called Megrez in the Great Bear, or Plow, the star at the root of the Great Bear's tail ; this star lies due South of the Pole Star on March 21st at midnight.
The third star to complete your clock-hand lies above the Pole Star, between it and your imaginary figure 12 midnight on the clock-face, and this is the star called Caph. It is the most westerly star of the five that form the constellation, or group of stars, named Cassiopeia, sometimes called from its shape the Celestial Doubleyou (W). It is an easy group to spot in the sky, a slightly irregular W in formation.
Those three stars are always in a straight line all the year round; but on March 21st, your Zero, that straight line runs due North and South, or from 12 P.M. to 12 A.M. on your imaginary clock-face, when the time by your watch is exactly 12 midnight. Now how can you tell the time on other nights of the year?
You must know, and remember, that the hand of your clock moves anti-clockwise at the rate of four minutes in every 24 hours, or about two hours a month (4 times 30 days equals 120 minutes, or 2 hours).
Suppose, now, that you are out night hiking on July 21st and have left your watch at home. You will calculate that it is four months from Zero, which means eight hours. Spot your clock-hand, or pointer, in the sky, and mark what hour on your clock-face the hand seems to point to; you may find it pointing to the hour Of 7 A.m. Now subtract from 7 A.M. the number of date-hours, 8; or, in other words, move your pointer back (clockwise) 8 hours from 7 A.m. That will bring it to 11 P.M. on your dock-face; and 11 P.M. will be the correct time.
Simple, isn't it? Spot what hour your pointer seems to indicate, calculate the number of date hours (at the rate of 2 hours per month and 4 minutes per day from March 21st), and move the pointer back that number of hours. The answer will be the right time. Do you want another example ?
Suppose the date is November 14th, so the number of date-hours will be about 151. Your pointer is pointing a bit short of 5 P.M. Move it backwards 15 hours from 4 P.M. (the same as 151, hours from 4.45 P.M.) and you will find it would be at 1 A.M. So the true time is 1 A.M. in the morning of November 5th.
Summer Time
One more thing to remember is that this star-clock gives true [Standard] time, but in the summer months you live by Summer Time [Daylight Savings Time]. So between the second Sunday in April and the first Sunday in October you must, after you have made your calculation of the true time by the stars, add one hour to find the actual working time of night. Thus, in the first example given above, for July 21st, the true time you found to be 11 P.M., but the time by your watch is 12 midnight.
So don't miss your last train by forgetting to make this little adjustment! But remember that it is not needed in the winter months, because in October we simply put our clocks and watches back to true time.
This method of telling the time by the stars has one great advantage over all others, and there are several others. That advantage is, that all the three stars named are visible above the horizon in the Northern Hemisphere all the year round, provided they are not hidden by cloud or fog.
As you may know, some constellations dip below the horizon on a certain date each year, and are invisible for a period of weeks or months. For instance the constellation or group named Orion, which is one of the groups useful for spotting the Pole Star, is not available between about 7 P.M. on May 20th and about 5 A.M. on July 20th, supposing you could see any stars at all on those dates at those times. So you cannot rely on Orion for telling the time during that period at least.
But as long as clouds or fog do not prevent your seeing the Pole Star, the Great Bear, and Cassiopeia, you will be able to tell the time at night by the stars.
